Method of treating steel to produce a fine-grained condition



United States Patent 3,201,288 I METHOD 9F TREATING STEEL Tfi PRGDUQE A FlhlE-GRAENED QQNEITKUN Raymond A. Grange, Washington Township, Westnioreland County, Pa., assignor to United States Steel Corporation, a corporation of New Jersey No Drawing. Filed Nov. 1, 1953, Ser. No. 32tl,837

Claims. (Cl. 148-424) This application is a continuation-in-part of my application Serial No. 849,715, filed October 30, 1959, now abandoned, for Method of Treating Steel to Produce a Fine-Grained Condition.

This invention relates to a method of treating steel to develop a line grain size therein and more particularly to a method of producing an ultra-fine austenite grain size in steels produced either by line grain or coarse grain melting practice.

The advantages of a fine austenite grain size in steel for improving toughness and ductility without sacrifice of strength are well known. The conventional method of producing fine grain in steel is by deoxidizing with aluminum, aluminum plus silicon or one of the so-called complex deoxidizers containing additional elements such as vanadium, titanium, zirconium and colurnbium. While the resulting grain size is in the 5 to 3 range on the ASTM scale, the use of such deoxidizers may result in inclusions in the steel which are undesirable. Steels deoxidized by silicon alone are less subject to such inclusions but are likely to have a relatively coarse grain size which may be improved to some extent by subsequent processing or heat treatment but with variable results due to the tendency of the austenite grains of such steels to coarsen.

It is accordingly an object of the present invention to produce a line-grained condition in steels melted by either fine or coarse grained practice.

It is a further object to consistently and commercially produce a finer grain in steels than was heretofore possible.

I have discovered a method for producing very fine grains in steel made either to fine-grained or to coarsegrained melting practice, which method consists essentially of deformation and recrystallization by the steel while in an austenitic condition. To retain the beneficial effect of the ultra-fine grain size produced by my improved treatment, the treatment must be followed in direct sequence by cooling in such manner as to cause the austenite to transform to a microstructure having the desired properties. This may involve furnace cooling, air cooling or quenching from the recrystallizing temperature. Any subsequent heat treatment which re-austenitizes the steel must be avoided as such would enlarge the austenite grains and destroy all benefits of the prior fine grain processing of my invention.

In accordance with the teachings of my invention, steel is preferably heated to substantially the lowest temperature at which it becomes completely austenitic but with a high heating rate may be heated to within about 300 F. above the lowest temperature at which the steel is completely austenit-ic provided the steel is deformed before any substantial grain growth occurs. Such temperature range varies for different types of steel and generally ranges from 1460 to 170il F. The steel, prior to any substantial grain growth, is then, while at such temperature, deformed to reduce the cross-sectional area thereof at least 25% and preferably about 50% or more, by any suitable method such as rolling, forging, extrusion or drawing. In any event, this reduction must be performed rapidly with the least possible loss of time so that the reduction is complete before the steel starts to recrystallize. It may be either in a single operation or in quickly chamber wherein it is held for sufiicient time :at a ten perature close to the prior austenitizing temperature to recrystallize it. This recrystallization produces very fine equiaxed grains. The recrystallization temperature should be no higher than the austenitizing temperature and preferably is slightly lower. The lower limit for recrystallization is the minimum temperature at which deformed austenite begins transformation. This is generally at or slightly below the equilibrium transformation ten1perature and will vary depending on the composition of the steel but is generally within the range of l2ll0 to 1606 F. The time required for complete recrystallization also varies from steel to steel and is influenced by the term perature and amount of deformation. However, it is very important that this time be at most very little more than is required for complete recrystallization since longer holding produces grain growth. Changes in austenite grain size during processing by my method can be de termined by suitable metallographic means and the minimum time for complete recrystallization thus determined.

In conventional hot rolling or forging, steel is heated to a high temperature, usually in the range .2200 to 2400 F., whereupon austenite grains become large. It is then deformed to the desired shape, the temperature of the steel falling during the hot working but usually being above 1700 F. when deformation is completed. This leads to relatively coarse grains in the product which is then reheated to refine the grain size. When steel is processed according to the teaching of this disclosure, coarse austenite grains are avoided in the first place by heating only to substantially the lowest temperature that will make the steel completely austenitic or rapidly heating it to within about 300 F. thereabove provided the steel is deformed before any substantial grain growth occurs. Deformation at this relatively low temperature olongates the comparatively small grains and renders them unstable so that they will, on subsequent holding at the proper temperatures recrystallize to an even finer grain size.

In producing the teaching of my invention, the finest grain size will be obtained in steels to which special elements, such as aluminum, have been added to inhibit grain coarsening. Nevertheless, cheaper grades of steel made according to coarse-grained melting practice can be rocessed by my method to a grain size at least as line as results from conventional heat treatment of the socalled fine-grained steels.

To further illustrate the teachings of my invention, reference is made to the following specific examples:

Example A An (Lil-inch thick plate of aluminum killed 51B-60 steel containing Percent Carbon 0.64 Manganese (1.88 Silicon 0.28 Chromium 0.83 Boron 0.0006

was austenitized at l700 by heating in a furnace. it was then rolled before any substantial grain growth had occurred in a single pass to 6.03-inch thick sheet. From the rolling mill, it was quickly transferred. to a second furnace at 1500 F. and held therein for 5 minutes after which the sheet was quenched in oil. The resulting austenite grain size. was #11 on the ASTM scale. By conventional heat treatment the finest grain size obtainable in this same steel would be ASTM #8.

3 Example B An 0.11-inch thick plate of aluminum killed SAE 1040 steel containing Percent Carbon 0.40 Manganese 0.89 Silicon 0.17

was austenitized at 1500 F. and then rolled to 0.03-inch thick sheet in one pass. From the rolling mill it was quickly transferred back into the furnace at 1500 F. and held for seconds, after which time it was removed and cooled in air. The resulting austenite grain size was #14 ASTM, as compared to a grain size of about #8 for the same steel normalized from 1500 F.

These two examples demonstrate the very marked degree of grain refinement which results from processing steel in accordance with my process. An austenite grain size of #11 ASTM is rarely encountered in steel and one as small as #14 has never, to my knowledge, been produced heretofore in any commercial steel.

The process described above involving starting with cold steel of such a shape that at least 25% deformation is required to reduce it to the desired dimensions and then heating it to the temperature range of 1400 to 1700 F. will produce uniform results. However, it is sometimes possible to roll or forge some steels in a conventional way, cool them only to the 1400 to 1700 F. range, and then deform and recrystallize them in accordance with my process. This avoids the expense and time involved in reheating. In a few instances, the rate of recrystallization may be sufficiently rapid that delaying the quench of air cooling will provide sufficient time for recrystallization if the deformation has been properly accomplished.

While I have shown and described several specific embodiments of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of my invention, as defined in the appended claims.

I claim:

1. A method of producing ultra-fine austenite grain size in steel comprising austenitizing the steel at substantially the lowest temperature at which it is completely austenitic, deforming said steel while at such temperature to reduce the cross-sectional area thereof at least 25 at a sufiiciently rapid rate that deformation is completed prior to any recrystallization to produce elongated austenite grains therein and then holding said steel at a temperature below the said austenitizing temperature and above about its equilibrium transformation temperature to substantially recrystallize it prior to any transformation, the time at the recrystallizing temperature being just suificient to complete said recrystallization thereby to produce equiaxed austenite grains therein having an austenite grain size no larger than ASTM #11.

2. A method of producing ultra-fine grain size in steel below the austenitizing temperature and above about its equilibrium transformation temperature to substantially recrystallize it prior to any transformation, the time at the recrystallizing temperature being just sufficient to complete said recrystallization thereby to produce equiaxed austenite grains therein having a grain size no larger than ASTM #11 and then quenching said steel to transform it to a fine grained martensitic structure.

3. A method of producing ultra-fine austenite grain size in steel comprising austenitizing the steel at between the lowest temperature at which it is completely austenitic and 300 F. thereabove, deforming said steel while at such temperature to reduce the cross-sectional area thereof at least 25 at a sufficiently rapid rate that deformation is completed prior to any recrystallization to produce elongated austenite grains therein and then holding said steel at a temperature below the said austenitizing temperature and above about its equilibrium transformation temperature to substantially recrystallize it prior to any transformation, the time at the recrystallizing temperature being just sufficient to complete said recrystallization thereby to produce equiaxed austenite grains therein having an austenite grain size no larger than ASTM #11.

4. A method according to claim 3 wherein the steel is austenitized at a rapid rate and is deformed before any substantial grain growth occurs.

5. A method of producing ultra-fine grain size in steel comprising austenitizing the steel at between the lowest temperature at which it is completely austenitic and 300 F. thereabove, deforming said steel while at such temperature to reduce the cross-sectional area thereof at least 25% at a sufficiently rapid rate that deformation is completed prior to any recrystallization to produce elongated austenite grains therein, then holding said steel at a temperature below the austenitizing temperature and above about its equilibrium transformation temperature to substantially recrystallize it prior to any transformation, the time at the recrystallizing temperature being just sufficient to complete said recrystallization thereby to produce equiaxed austenite grains therein having a grain size no larger than ASTM #11 and then quenching said steel to transform it to a fine grained martensitic structure.

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK, Primary Examiner. 

3. A METHOD OF PRODUCING ULTRA-FINE AUSTENITE GRAIN SIZE IN A STEEL COMPRISING AUSTENITIZING THE STEEL AT BETWEEN THE LOWEST TEMPERATURE AT WHICH IT IS COMPLETELY AUSTENITIC AND 300*F. THEREABOVE, DEFORMING SAID STEEL WHILE AT SUCH TEMPERATURE TO REDUCE THE CROSS-SECTIONAL AREA THEREOF AT LEAST 25% AT A SUFFICIENTLY RAPID RATE THAT DEFORMATION IS COMPLETED PRIOR TO ANY RECRYSTALLIZATION TO PRODUCE ELONGATED AUSTENITE GRAINS THEREIN AND THEN HOLDING SAID STEEL AT A TEMPERATURE BELOW THE SAID AUSTENITIZING TEMPERATURE AND ABOVE ABOUT ITS EQUILIBRIUM TRANSFORMATIONM TEMPERATURE TO SUBSTANTIALLY RECRYSTALLIZE IT PRIOR TO ANY TRANSFORMATION, THE TIME AT THE RECRYSTALLIZING TEMPERATURE BEING JUST SUFFICIENT TO COMPLETE SAID RECRYSTALLIZATION THEREBY TO PRODUCE EQUIAXED AUSTENITE GRAINS THEREIN HAVING AN AUSTENITE GRAIN SIZE NO LARGER THAN ASTM #11. 